osdmaptool is a utility that lets you create, view, and manipulate
OSD cluster maps from the Ceph distributed storage system. Notably, it
lets you extract the embedded CRUSH map or import a new CRUSH map.

will create a relatively generic OSD map with the numosd devices.
If –pgbits is specified, the initial placement group counts will
be set with bitsperosd bits per OSD. That is, the pg_num map
attribute will be set to numosd shifted by bitsperosd.